Foams of high molecular weight polycarbonates are known (see, for example, DE-AS 1,031,507). Suitable blowing agents are, for example, inert gases such as N.sub.2 or CO.sub.2 or compounds which split off CO.sub.2.
In the past, it was widely held that thermoplastic polycarbonates could only be processed satisfactorily in a completely dry state and that the presence of moisture upwards of a certain quantity led to molecular weight degradation and the formation of blisters (see, for example, Taschenbuch Bayer-Kunststoffe, 2nd Edition 1959, page 177, Plastics Technologie, 10, 1964, pages 32-36 and GB-PS 841,652).
This sensitivity to moisture of thermoplastic polycarbonates does not exist in certain cases, for example if the moisture is introduced into the thermoplastic polycarbonate as an aqueous size for glass fibers or if the moisture is added for the removal of residues of solvent from the polycarbonate and subsequently expel led together with the solvent residues in an evaporation extruder (see U.S. Pat. Nos. 3,577,378, 3,567,813 and EP-A2-0,300,485).
It was an object of the present invention to choose the operating conditions for the incorporation of Al.sub.2 O.sub.3 (H.sub.2 O).sub.3 and subsequent molding of the mixture of polycarbonate with Al.sub.2 O.sub.3 (H.sub.2 O).sub.3 so that controlled degradation of the polycarbonate will take place which will bring about foaming without too seriously impairing the mechanical and other properties of the thermoplastic polycarbonate and of the molded foam products obtainable therefrom according to the invention. This has been achieved by the operating conditions according to the invention, which could not have been deduced from the state of the art .